For one of my next rig builds I'm planning on using a 3.9 cummins diesel. I never see people running low axle gears with diesels due to the very low RPM range. However for crawling I wonder if a diesel could benefit from some low gears. So, I was thinking about doublers. Say, run 3.50 in the axles with a 203/205 combo. Would it be worth it for crawling, or would I find that I'd never use the 4 to 1 range?

For one of my next rig builds I'm planning on using a 3.9 cummins diesel. I never see people running low axle gears with diesels due to the very low RPM range. However for crawling I wonder if a diesel could benefit from some low gears. So, I was thinking about doublers. Say, run 3.50 in the axles with a 203/205 combo. Would it be worth it for crawling, or would I find that I'd never use the 4 to 1 range?

I think it would be way overkill! With minimal work its possible to get a 4bt to around 200hp, 400lb-tq. Alot of people that I know that do "michigan wheeling" 4 to 1 low range is very deep and almost useless. The deepest i would go would be 3.0 to 1 or a standard 2.72 to 1 t case ratio and just put 5.13's in the axles or 5.38's, depending on tire size.

There is a Jeep build on pirate where the guy installed a 4bt into his TJ, i think he was using a 3.8 atlas.

I think it would be way overkill! With minimal work its possible to get a 4bt to around 200hp, 400lb-tq. Alot of people that I know that do "michigan wheeling" 4 to 1 low range is very deep and almost useless. The deepest i would go would be 3.0 to 1 or a standard 2.72 to 1 t case ratio and just put 5.13's in the axles or 5.38's, depending on tire size.

A 4bt should have enough torque to move a decent sized rig over obstacles at an idle so extremely low gearing would be unnecessary. Kind of the same idea when you try to stall a diesel and it lugs down and keeps pulling. If you are still not sure, do what I did on my YJ. Run a 203 t-case and if you still want a doubler you at least have the front half done. And many times that is the harder part, mating the trans to the 203 range box.

the problem with diesels are that they dont have enough snappy power. many times you will have a foot or even inches to get a decent launch to bump over an obstacle. even by break torquing you will not get as smooth and fast of a response as a gas powered engine. once they are in the power the torque curve is phenominal, low speed crawling the torque is great.. they just always seem to miss the middle.

Deep axles isn't an option, and neither is building up the motor. I plan to make this thing driveable. That's why I'm thinking doubler.

unless your running stickies in moab i doubt youll ever really see the potential of the doubler. also running a manual transmission and doubler can be tricky to get the right ratio when crawling. On paper it looks like the possibilitys are endless but you always find yourself needing to shift at the worst times.

5spd 4.10-4.56 a set of 37's, a standard t-case ratio and call it good.